Various types of applicators are known. One of these applicator devices is an extrusion-type applicator, which has a doctor edge portion and which is used in various fields such as those mentioned in the Japan Patent Applications (OPI) Nos. 138036/75, 84771/82 and 104666/83 and Japan Patent Application No. 7306/78. (The term "OPI" as used herein means an "unexamined published application.") However, these extrusion-type applicators have a common disadvantage in that each of the devices has only a very narrow operating range providing proper application. For example, it is very difficult to repeatedly perform the application of an applied liquid film having a thickness of 20 micrometers (.mu.m) or less, particularly when the linear speed of the carrier is greater than 100 to 150 meters per minute (m/min). In these applicators, the quantity of air which is entrained between the applicator head and the running carrier increases sharply as the speed of application is increased. In order to apply a liquid uniformly in the form of a thin film on the carrier when the speed of application is greater than 100 to 150 m/min., the pressure of the liquid at the outlet portion of the slot of the applicator head, or a similar opening, needs to be increased appropriately in order to avoid the adverse effects of air entrainment. Therefore, if the pressure of the liquid cannot be made high enough, the thickness of the applied liquid film becomes nonuniform due to the mixing of air bubbles into the film.
Japan Patent Application (OPI) No. 10466/83 discloses an applicator shown in FIG. 6, which was proposed in an attempt to increase the pressure of a liquid during the application period to increase the speed of application without allowing the thickness of the applied liquid film on the carrier to become nonuniform in the direction of the width of the carrier. Since the pressure of the liquid can be made high locally at the surface of the slot edge of the doctor edge portion of an applicator head and kept relatively uniform, air is prevented from entering the applied liquid film from the surface of the carrier at the back edge portion of the head, which makes it possible to rapidly apply the liquid to the carrier, thus creating a thin film having a uniform thickness.
However, making the speed of application high while simultaneously making the pressure of the liquid high, as discussed above, results in a problem in that the edge portion of the applicator head is likely to be chipped. It will be appreciated that this causes very serious defects in the applied liquid film on the carrier such as streaks To cope with this problem, the applicator disclosed in the Japan Patent Application No. 85672/89 was proposed. This applicator head is made of a material less likely to be chipped even when the pressure of the applied liquid is high or when the linear speed of the carrier is high. Therefore, the possibility of a defect in the applied liquid film is lowered to some extent.
However, when the applied liquid is a magnetic liquid in which a ferromagnetic iron oxide powder or other abrasive grains are dispersed, when the speed of application is very high and when the thickness of the applied liquid film is small, the pressure effects and abrading action of the powder or grains during application of the liquid film have made the problem more conspicuous.
The applicator disclosed in the Japan Patent Application (OPI) No. 35959/90 includes a sloping plane, having a relatively large width of 0.01 to 1 millimeter (mm) on the doctor edge portion of the applicator head, on the upstream side of the portion with respect to the running direction of the carrier, for the purpose of preventing extraneous substances from being trapped on the portion. Although the purpose is achieved, the device is not effective in solving the chipping problem discussed above. Therefore, when a liquid is applied continuously to the carrier by the device at a high speed for a long time period, a streak is made on the surface of the applied liquid film on the carrier. It will be apparent that this adversely affects the signal-to-noise ratio or carrier-to-noise ratio of a magnetic recording medium manufactured from the liquid and the carrier.
The present invention was motivated by a desire to solve this problem.